This invention relates to a method of monitoring a gas discharge lamp, in which method an electrical test value (M) relating to the lamp voltage is detected, as well as to an arrangement for monitoring a gas discharge lamp comprising a test circuit for detecting an electrical test value relating to the lamp voltage.
The end of life of a high-pressure gas discharge lamp with an internal lamp pressure of more than 200 bar (for example a UHP lamp) is often characterized by an explosion of the lamp bulb. Such UHP lamps consist substantially of an approximately spherical quartz bulb in which tungsten electrodes are laterally sealed in so as to have an interspacing of 1.0 to 1.3 mm. A gas discharge is built up between these electrodes which, in combination with an upward convection flow, can bring the upper bulb surface to temperatures close to the melting point of the quartz from which the bulb wall is made. The electrode spacing increases during lamp life owing to the removal of tungsten from the electrode tips. Since the lamps are controlled to a constant power, the voltage in the lamp (the burning voltage) increases. In the case of an explosion, the housing parts surrounding the lamp are contaminated with glass fragments and materials present in the interior of the lamp (for example, mercury and other filling ingredients). This is why known lamps are preferably accommodated in closed reflectors.
The incorporation of a lamp of high power in a closed reflector is very difficult in small reflector systems on account of thermal aspects. The use of a pierced front plate with the free lamp end outside the reflector for reducing the thermal problems has the disadvantage that some explosion products may leave the reflector and cause damage inside the housing (for example, a projector).
The German patent document laid open to public inspection DE 197 15 254 discloses the use of a ballast designed for the operation of gas discharge lamps also for the purpose of monitoring lamp life. To detect lamp deterioration, and to render possible a timely exchange of lamps in the case of high-pressure gas discharge lamps, the ballast is provided with a monitoring circuit which detects a rise in the burning voltage (i.e. stable operating voltage) of the discharge lamp for a longer period in excess of a limit value and which switches off the ballast when this is necessary. The ballast comprises the functions necessary for operating a gas discharge lamp in known manner (for example, providing the operating power and an ignition device). In addition, the ballast comprises a monitoring unit (control circuit) with a monitoring circuit for the burning voltage and a timer circuit. The monitoring circuit detects the voltage across the gas discharge or some other voltage related thereto. When the burning voltage exceeds a programmable limit value, the monitoring circuit issues a signal at its output. This signal does not immediately serve to block or inactivate the supply source of the lamp, but it is first tested for its time gradient by means of the timer circuit. Once the signal has persisted for a given time period, the supply source is switched off.
Methods and arrangements for monitoring gas discharge lamps in accordance with the present state of the art monitor the burning voltage of the lamp only as regards its rise until a limit value is reached. Such methods are useful for those lamps whose burning voltage rises through lamp life because of deteriorating electrodes. The passing of a limit value can be used as the switch-off signal so as to avoid a destruction of the lamp. Often gas discharge lamps do not achieve the maximum lamp life characterized by a maximum voltage because the lamp is destroyed earlier by other causes. Thus blackening at the inside of the discharge vessel (lamp bulb) arises before the maximum voltage is reached, in particular in the case of an imperfect filling of the lamp. This leads to a volume increase of the lamp bulb (continuous inflation), and thus to a pressure drop. Since the burning voltage does not rise any further because of the pressure drop, in spite of a further increase in the electrode spacing, the risk of lamp explosion cannot be detected by known monitoring devices.
It is accordingly an object of the invention to better monitor the operation of gas discharge lamps. In particular, to better predict lamp life and explosion risks.
The object of the invention is achieved in that a warning signal is generated when a test value relating to a clear decrease in the lamp voltage is detected. The electrical test value may either be proportional (for example, a voltage outside the lamp which can be measured in a simple manner and which preferably is directly proportional to the lamp voltage), or inversely proportional (for example, the current supplied to the lamp) to the stable operating lamp voltage or burning voltage. The method proposed here, in contrast to known methods, does not detect a rise or a gradient which is substantially constant in time, but a gradient in time of the lamp voltage with a negative tendency. The clear decrease may be detected in particular on the basis of a characteristic negative tendency of the lamp voltage. As an additional characteristic for the time gradient of the lamp voltage, for example, the occurrence of a negative slope over a certain period, a given, adjustable absolute value, or a combination of the two may be utilized.
It is provided in an embodiment of the invention that the test value is compared with a reference value, that a differential value corresponding to the deviation of the test value from the reference value is formed for each test value, and that a warning signal is generated when a test value corresponding to an adjustable maximum differential value is detected. The reference value is time-dependent because it preferably corresponds to a standard value expected for the relevant lamp at a given moment in time. Such standard values may be derived from measurements of similar lamps. A further possibility is a decision to generate the warning signal on the basis of a detected minimum value of the lamp voltage. This minimum value may also be coupled to the relevant reference value with time dependence. The minimum value corresponds to a minimum of the lamp voltage at which the lamp has to be switched off and replaced for reasons of safety.
In a preferred further embodiment of the invention, it is provided that a microprocessor contained in a ballast unit suitable for operating the gas discharge lamp receives the test value, carries out the detection of the test value corresponding to a clear decrease in the lamp voltage, and generates the warning signal when such a test value is detected. A microprocessor is usually present in known electronic ballast used for operating gas discharge lamps, which microprocessor controls the ignition voltage, frequency, pulse height, and power and which switches the lamp back to half power in the case of a low supply voltage. This microprocessor is usually constructed or may be modified in a simple manner such that it can carry out the additional function of monitoring the gas discharge lamp in accordance with the invention.
In an advantageous embodiment of the method according to the invention, the microprocessor receives the test value at regular time intervals, does not utilize the test values received during an adjustable burning-in phase of the gas discharge lamp for determining the test value corresponding to a clear reduction in lamp voltage, determines the rise of the test value from five consecutive test values each time, and generates the warning signal when a negative gradient of the test value is detected. These functions can be carried out by a usual microprocessor. The microprocessor receives a test value at moments which can be set by means of the supply frequency and a counter, and deposits it in a memory (for example a register). A time for the burning-in phase of the lamp can be set in a similar manner. The first hours are characterized by a burning-in phase of the lamp, in which voltage fluctuations may occur which should not be taken into account, so that a first test value is not stored until after this period (for example 2000 h). Once five test values have been stored in the memory, the microprocessor determines the gradient of the test value by means of usual mathematical processes. The number of the test values and their frequency in time may obviously be adapted in dependence on the required accuracy and on the power of the microprocessor used. When a negative gradient is detected, an output of the microprocessor becomes active, and the warning signal is generated thereby.
Furthermore, the object of the invention is achieved by means of an arrangement in which the control circuit is designed for generating a warning signal when a clear reduction in the test value corresponding to the lamp voltage is detected. The control circuit may be realized, for example, by a suitably adjusted microprocessor, a logic circuit with fixed wiring, or in analog circuitry. Advantageous embodiments of the device according to the invention are indicated in the dependent claims.
The method and arrangement according to the invention for monitoring a gas discharge lamp render possible a highly reliable prediction of explosions and can thus achieve a timely switching-off of the lamp. It is immaterial here whether the thermal load is caused by processes inside the lamp (for example, recrystallization owing to blackening) or also partly by external loads (for example, failure of the appliance cooling). The burning voltage of the discharge lamp refers to the (stable) operating voltage of the discharge lamp which is present subsequent to the ignition phase of the lamp.